PHYS 5326 – Lecture #20

Wednesday, Apr. 25, 2007 PHYS 5326, Spring 2007Jae Yu

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PHYS 5326 – Lecture #20Wednesday, Apr. 25, 2007

Dr. Jae Yu

•Higgs Search Strategy•Higgs Search Channels•Backgrounds to Higgs Searches•Requirement on Experiments for Higgs Searches


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Hadron Collider SM Higgs Production

MH=130 GeV


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Tevatron Run I Results• Run I limits are not very stringent

Channel D (95% CL) CDF(95% CL) Theory

WH l bb <28 pb <27 pb 0.07

ZH bb <8 pb 0.10

ZH ll bb <7.5 pb 0.11

No real mass limit here…


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Tevatron Run II Higgs Physics• BR @ 2 TeV

– Use associated H+W/Z production• H bb

(pb) for MH=100 GeV

gg H 1.17

qq,gg WH 0.31

qq,gg ZH 0.17

qq,gg H+2jets 0.12

All others <0.02

BackgroundsWZ+ZZ 4.4

Wbb+Zbb 14

tt 7.5

tb+tq_tbq 3.4

QCD O(106) dijetO(105) 4jet

W e 2800


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MH=130 GeV

For MH<130GeV• H bb• H WW• H gg or • H cc

• H WW/ZZ• H bb• H gg or • H cc

For MH>130GeV


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H (h) bb• Two b-quark jets in the final state• Look for signature of b-quark jets

– b-quark jets contain B mesons– B-mesons have finite lifetime and decay in flight

• Final state of interest– A displaced vertex from the primary vertex– Existence of secondary leptons from b-quark

semileptonic decay


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How does a displaced vertex look?

b vertex

Silicon

Detectors

Beampipe

2” at Run II1” at present


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H (h) WW• W’s decay immediately • Look for signature of W’s

– W branching ratios• ~11% to leptonic final state (l+) for each lepton• ~69% to quark and anti-quark final states

• Final states of interest– One high PT lepton (e or ) + missing ET + two jets

– Two opposite charge high PT leptons (ee, , or e) + Missing ET

– Four jets


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H (h) ’s decay almost immediately • Look for signature of ’s

– branching ratios• ~17% to leptonic final state (e,+) for each lepton• ~68% to hadrons (one or three charged hadrons)

• Final states of interest– One medium PT lepton (e or ) + missing ET + one

narrow jet– Two opposite charge medium PT leptons (ee, , or

e) + Missing ET

– Two narrow jets


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For HW or HZ• Cross section is about 10% of ggH• Both H and W/Z decay immediately• Look for signature of vector bosons with H decay

final states• W or Z final state characteristics

– Decay to Leptons or quark jets• Final states of interest for HW

– Higgs decay + One high PT lepton – Higgs decay + two light quark jets


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HZ Final States• Z decay branching ratio

– ~3% for each charged lepton pairs– ~20% two neutrino final states– ~70% quark, anti-quark final states

• ~15% bb final states

• Final states of interest for HZ– Two high PT lepton + Higgs decay channels

– Large Missing ET + Higgs decay channels– Four b-jet final states


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Summary of Final States of Interest• W decay:

– W qq channel very hard• depends on dijet mass resolution

– W e, , • Isolated lepton plus missing ET

• Z decay– Z qq same as W – difficult– Z ee, (probably not )

• Isolated lepton plus mass resolution

• Higgs decay:– Look for bb pairs

• Impact parameter resolution• Silicon vertexing• Di-jet mass resolution

b-tagging is a MUSTb-tagging is a MUST


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• WH l bb backgrounds – Wbb

• Where the W decays leptonically• Cross section is 2.3nb*s

2*mb-threshold factor for each lepton

– WZ: • W decays leptonically while Z decays to bb• Cross section is 2.3nb*EW2*b final state branching ratio

– tt: Each t decays to W and b pairs. How would this become a background?

– single t: How?• WH qq bb overwhelmed by QCD background since it has

four jets in the final state

Higgs Backgrounds To WH


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• ZH l l bb final state for signature– Zbb

• Z decays to two leptons• Cross section is 0.2nb*s

2*mb-threshold factor for each lepton

– ZZ• One Z decays to two leptons while the other decays to bb • Cross section is 2*0.2nb*EW*b branching ratio

– tt:How does this channel become background to ZH?

Higgs Backgrounds to ZH


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• ZH bb – QCD:How would this become background to this process?– Zbb: Z decays to neutrinos– ZZ: One Z decays to neutrinos while the other decays to

bb– tt: How would this process become background?

Higgs Backgrounds to ZH


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• WW final state is a generic, inherent and irreducible background

• 2l+MET final state– ZZ: One Z decays to two leptons while the other to two

neutrinos– Drell-Yan (qq*ll)– tt: How this could be the background to this process?– WZ: Z decays to two leptons, while W decays to l+ where l

is lost : Both ’s decay leptonically

Higgs Backgrounds to gg H WW*


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• 1l+MET+2jets final state– WZ: W decays to l+ and Z decays to two jets– ZZ: One Z decays to two leptons (of which one is lost) while

the other to two jets– tt : How is this become a background to this process?– tW:How could this become a background to this process?

Higgs Backgrounds to gg H WW*


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• The best discovery channel is with HW and HZ• The background for mH < 130 GeV are

– WH l bb backgrounds Wbb, WZ, tt, single t– WH qq bb overwhelmed by QCD background since it

has four jets in the final state– ZH l l bb backgrounds Zbb, ZZ, tt– ZH bb backgrounds QCD, Zbb, ZZ, tt

• The background for mH > 130 GeV are– gg H WW* backgrounds Drell-Yan, WW, WZ, ZZ, tt,

tW,

Summary of Higgs Physics Backgrounds


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Tevatron Higgs Production


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TeVatron Run II Higgs S/N• H bb

– S/N drops as MH increases• Run out of events….• Ultimately limits the experimental reach

• H WW/ZZ (virtual)– Depends on di-jet mass resolution

1 fb-1 delivered

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PHYS 5326 – Lecture #20